Data by month

Seems like you're trying to summarize your data, if so, try the approach shown in the example attached.

My suggestion would be to use a purpose built Pivot Table for this:

"Date" as Column Label and Group by Month etc
Calculated Field ["Duration" which is 'End Time' - 'Start Time'] as Data Field
(set to SUM format as [h]:mm)

Attached is (XL2007) working example of the above - you can modify the layout of course.

If you just wanted the total of hours, add column D as a helper to your first sheet :-

=Month(a1)

then you can total for each month with :-

=SUMIF(Sheet1!$D$2:$D$13,9,Sheet1!$C$2:$C$13)-SUMIF(Sheet1!$D$2:$D$13,9,Sheet1!$B$2:$B$13)

Pivot table is the best way to go!!!!!!
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squiggler - if we're championing helpers it would make most sense to store both month and difference so as to negate the need for two SUMIFs, no ?

I dont think I was championing helpers, Pivot table is as I said the best option!

But yes you are correct!

Although, given the inherent smallness of the data,Personally I would have employed a sumproduct for simplicity, given a large dataset pivot table wins out without doubt!

I wasn't debating the merits of the various approaches... simply making the point that if anyone opts for helpers then they may as well use them to their max. advantage [in terms of simplifying final calculation(s)]